Apparently the best thing to shout is "Fire!". Some will not answer calls for generic help because they're afraid to get involved, or think it's people messing about - but "Fire!" draws more active attention in case it will directly affect the audience.

I don't know if this is true, or even if I want it to be true (it's a bit cynical about human nature, isn't it?), and I certainly doubt my own presence of mind to remember such advice were I ever to find myself in trouble, but I thought I'd share.